1. Field of the Invention
The present invention relates to the area for touch screens, and more particularly to techniques for detecting multiple touch points on a touch screen.
2. Description of Related Art
Touch screens are gradually becoming main means of inputting information to an electronic system. Techniques for detecting multiple touch points are more attractive in the touch screen fields. However, techniques for detecting multiple touch points are mostly implemented on an optical touch screen, which are too expensive for most customers.
FIG. 1A is a schematic diagram showing a conventional four-wire resistive touch screen with a single touch point thereon. The four-wire resistive touch screen includes an X conductive layer and a Y conductive layer. The X conductive layer has a positive terminal Xp and a negative terminal Xn at two respective ends thereof. The Y conductive layer has a positive terminal Yp and a negative terminal Yn at two respective ends thereof. The X conductive layer is physically separated from the Y conductive layer by a spacer. When a touch point P1 with enough pressure appears on the resistive touch screen, the X conductive layer may contact with the Y conductive layer at the touching point P1. FIG. 1B is a circuit diagram showing an equivalent electrical circuit to the resistive touch screen shown in FIG. 1A.
To facilitate the description of the embodiment as shown, Xplate denotes a total resistance of the X conductive layer from the negative terminal Xn to the positive terminal Xp. The touching point P1 divides the total resistance Xplate into the resistances R1 and R3 proportionally as the X conductive layer has a uniform linear resistivity. Yplate denotes a total resistance of the Y conductive layer from the negative terminal YN to the positive terminal YP. The touch point P1 divides the total resistance Yplate into the resistances R4 and R6 proportionally as the Y conductive layer has a uniform linear resistivity. Rz denotes a contact resistance between the X conductive layer and the Y conductive layer at the touch point P1. Thus, the x and y coordinates of the touch point P1 may be located as long as the resistance values R3 and R6 are calculated.
The terminal Yp is connected to a positive reference voltage VT, the terminal Yn is grounded, and the voltage value V1 on the terminal Xp is measured. The voltage value V1 satisfies a following formula:
            V      ⁢                          ⁢      1        VT    =                    R        ⁢                                  ⁢        6            Yplate        .  
Similarly, the terminal Xp is connected to the positive reference voltage VT, the terminal Xn is grounded, and the voltage value V3 on the terminal Yp is measured. The voltage value V3 satisfies a following formula:
            V      ⁢                          ⁢      3        VT    =                    R        ⁢                                  ⁢        3            Yplate        .  
The resistance values R3 and R6 are calculated according to the above two formulas. So, the coordinates of the touch point P1 are located. However, the conventional four-wire resistive touch screen is mainly provided for detecting a single touch point on the touch screen. If there are two or more touch points on the touch screen, it is very difficult to determine a relative motion tendency of the two or more touch points by calculating the coordinates of each touch point.
Thus, improved techniques for method and device for detecting multiple touch points on a resistive touch screen are desired to overcome the above disadvantages.